1. Field of the Invention
The present invention relates to a lithium silicate glass ceramic material for the manufacturing of blocks and subsequent fabrication of single dental crowns with the aid of the CAD/CAM process. The invention relates to an improved version of such glass ceramic containing germanium dioxide to make it more castable with higher density and higher flexural strength than the lithium disilicate free of germanium dioxide.
2. Background Art
There are many products available in the market employing lithium disilicate material and covered by several U.S. patents. Some of these patents claim a process for the preparation of shaped translucent lithium disilicate glass ceramic products from a mixture of basic components (SiO2, Al2O3, K2O, Li2O, plus pigments and fluorescent oxides). The addition of lanthanum oxide results in a product with excellent dimensional stability on heating. Other patents describe the process for the production of lithium disilicate glass ceramic where mixtures of basic components except lanthanum oxide are claimed in different ranges. A patent also describes a lithium disilicate preparation which uses zirconium, titanium dioxide and phosphorus as nucleation agents in their formulation. There are also some other patents, scientific papers and technical books describing the preparation methods of lithium disilicate glass ceramic. Most of them use similar composition ranges of the patents described above and the thermal cycles of nucleation and crystallization.
Most of the existing patents in the dental field use the same basic components. The present invention uses germanium dioxide as a fundamental part of the formula. This oxide is broadly used in glass preparation for its good optical properties. The oxide has been well studied and has positive effects compared to common silicon glasses. It has been found that the addition of germanium oxide produces a melt with low viscosity facilitating the castability of the process and increases the thermal expansion and the refractive index of the resulting lithium silicate glass ceramic. More important, the addition of germanium dioxide increases the final density of the glass resulting in higher values of flexural strength than the lithium disilicate glasses free of germanium dioxide. Because the final composition of this invention uses a molar ratio of Si/Li between 1.8 and 1.9, only Lithium Silicate, instead of Lithium Di Silicate, is present as the main constituent of the crystalline phase after full crystallization of the glass ceramic.